If x-rays are used for an imaging method, these are typically generated in an x-ray tube and in the process irradiated from a relatively small surface to an anode, which, as a good approximation, can be considered as a punctiform x-ray source and is typically referred to as x-ray focus. In order to implement the imaging method, an x-ray detector is also frequently used, which is made up of regularly arranged detector elements or detector pixels.
When examining an object or patient using such a structure comprising an x-ray source and x-ray detector, the attenuation of the x-rays originating from the x-ray source, produced by the object and/or the patient, is generally to be detected directly toward the detector elements, for which reason a so-called anti scatter-grid, which is positioned on the detector elements, is also to be used in many cases.
A corresponding anti-scatter grid is in most instances made up of a plurality of walls, with which a honeycomb-type structure is realized. With the aid of this honeycomb-type structure, x-rays, which do not propagate along a straight connection between the x-ray focus and a detector element, are easily absorbed in the walls of the anti-scatter grid, whereas x-rays, which spread along a straight line between the x-ray focus and a detector element, reach the detector element without a relevant influence by means of the anti-scatter grid. To this end, the walls of the anti-scatter grid and in most instances also the detector elements, are aligned toward the x-ray focus.
Here a change in the focus position defines a change in the positioning of x-ray focus and x-ray detector, including anti-scatter grid, relative to one another, thereby virtually resulting in a misalignment of x-ray source and x-ray detector relative to one another and as a result in a reduction in the image quality which can be achieved using this arrangement. In order to prevent corresponding losses in quality, there is the possibility of either correcting the alignment of x-ray source and x-ray detector relative to one another or preparing the generated image data with the aid of a correction algorithm. In both instances, there is the need here to determine the position of the x-ray focus relative to the x-ray detector and thus also relative to the anti-scatter grid.
In the patent application US 2011/0176663 A1, an apparatus is described, with the aid of which the corresponding x-ray focus position can be determined. Here an anti-scatter grid is used, which is made up of a number of anti-scatter grid modules which are tilted in respect of one another in each instance. The relative alignment of the individual anti-scatter grid modules relative to one another is known and by evaluating the measuring signals of the detector elements assigned to the anti-scatter grid modules, it is determined which of the anti-scatter grid modules is best aligned relative to the x-ray focus.